Inside a ship, waste water from cleaning and/or from small leakages in the hull of the ship accumulates—typically at the bottom of the hull or in a sump provided for that purpose. Particularly in the engine room of the ship, waste water accumulates together with all kinds of contaminants in the bilge water sump. The predominant contaminants in the bilge water are fluid organic compounds, such as oil, grease, solvents, and/or fuel. Other contaminants may be solids, such as filings or swarf, or larger items, such as cans, lids, dropped tools, pieces of wood, paper, and/or textiles.
The above-mentioned fluid organic contaminants are hazardous to the health of staff working in a machinery space, since at least some of the contaminants may evaporate, creating carcinogenic vapours in the air of the machinery space. Furthermore, the contaminants are inflammable and have a tendency to concentrate at the surface of the bilge water, thereby causing a constant fire hazard. In case of fire, the contaminants develop hazardous gases and heavy smoke preventing any effective fire fighting with the additional danger of asphyxiation for those caught in the smoke. Such an accident is particularly serious when happening on a ship at sea, far from any harbour and easy rescue. In addition, the contaminants are potentially dangerous for the environment, as they pollute the sea, coasts, and harbours, when discharged directly into the sea.
The above-mentioned hazards arising from contaminated bilge water has been a problem well-known to generations of seamen and was and is often solved by pumping the bilge water directly out into the sea. However, international environmental regulations, such as the “International Convention for the Prevention of Pollution from Ships” (MARPOL 73/78), strictly regulate such discharge, among other things prohibiting any discharge of water containing more than 15 ppm oily contaminants and any discharge of oily contaminants at all within 50 miles of the nearest land.
In order to comply with these regulations, a number of oily water separators have therefore been proposed and are available on the market. However, the proposed systems are often very expensive, suffer from complexity, limited capacity at peak load, and/or lack of reliability, and have to be put out of operation frequently either for maintenance or due to system failure. As a consequence, the chosen solution is often still to simply discharge the contaminated bilge water directly into the sea disregarding all environmental regulations and the risk of serious punishment imposed on such illegal discharge by many countries.